Investigators in the Divisions of Clinical Pharmacology and Cardiology at Duke University Medical Center have used the Rankin Clinical Research Unit to study patients with atrial fibrillation and other common disorders of the heart rhythm. Atrial fibrillation is the most common disorder of the heart rhythm requiring treatment with antiarrhythmic drugs, and it accounts for over one-third of all U.S. hospitalizations for arrhythmias. Patients with atrial fibrillation have their usually normal heart rhythm replaced by periods of rapid, irregular heart beating that may cause a sense of pounding in the chest, shortness of breath, chest pain, dizziness, or loss of consciousness. These periods may last for minutes or hours, or they may last indefinitely unless acute treatment is given by a physician. AFFIRM is a multi-center, randomized clinical trial designed to compare 2 strategies for the management of patients with atrial fibrillation, the most common supraventricular arrhythmia. The first strategy is to use antiarrhythmic drugs to reduce the frequency of occurrences of atrial fibrillation; the alternate strategy is to use drugs to control the heart rate when atrial fibrillation occurs but to avoid antiarrhythmic drugs. Recruitment will continue until October 31,1999 and follow-up will continue for another two years after that. AFFIRM was initiated and is sponsored by the National Institutes of Health.